1. Field of the Invention
The present invention relates to a rotary actuator having a rotor which is rotatable relative to a stator.
2. Discussion of Prior Art
A rotary actuator has been utilized in association with a diverter gate in the sorting of mail or other items traveling by a conveyor. The rotary actuator is effective to rotate the diverter gate from one position to another position within a matter of a few milliseconds, typically within about 0.020 seconds, so as to permit a rapid sorting process. The angle of rotation of the diverter gate is typically about 15xc2x0 to 20xc2x0 to move the item of mail from one conveyor path to another conveyor path.
The angle of rotation through which the diverter gate is moved is limited by rubber stop bumpers. The rubber stop bumpers are mounted external to the rotary actuator so as to allow precise adjustment and to minimize impact noise by the diverter gate. At the end of its operating stroke, the diverter gate may tend to rebound as it impacts against one of the rubber bumpers.
If the diverter gate can rebound back into the previous conveyor flow path, a missortment or jam may occur. To prevent a missortment or jam from occurring, the flow rate of mail or other items must be decreased to give time for the diverter gate to return to its fully actuated position. Alternatively, the rate of operation of the rotary actuator must be decreased to reduce the kinetic energy of the rotary actuator and diverter gate at an end of stroke position. Of course, both of these solutions to the problem of diverter gate rebound are counter to rapid sorting requirements.
The rotary actuator for the diverter gate must provide for both rapid movement of the diverter gate from an unactuated position to an actuated position and holding of the diverter gate at its actuated position upon impact of the diverter gate against a rubber bumper. In order to provide both functions adequately, the starting torque of the rotary actuator must be high to provide a high diverter gate acceleration rate. The ending torque of the rotary actuator must be high to counteract the rebound energy imparted by the rubber bumper to the diverter gate.
Known rotary actuators have previously utilized either one of two basic design approaches. The first basic design approach utilizes a pole configuration termed as xe2x80x9cconstant air gapxe2x80x9d for the rotor and stator pole pieces. The second basic design approach utilizes a pole configuration termed as xe2x80x9cdiminishing air gapxe2x80x9d for the rotor and stator pole pieces. The air gaps are the working air gaps across which magnetic flux is conducted between the rotor and stator pole pieces.
The xe2x80x9cconstant air gapxe2x80x9d rotary actuator design is characterized by a high starting torque that decreases to a lower torque as the rotary actuator operates through its operating stroke (it being assumed that a constant current is applied to the coil of the rotary actuator). The high starting torque occurs when lobes of the rotor are only partially overlapping, or aligned with, corresponding stator lobes. Typically, there is a 3xc2x0 overlap of the rotor lobes and stator lobes at the initial starting position of the rotor.
The maximum torque for the xe2x80x9cconstant air gapxe2x80x9d rotary actuator design occurs between the initial position and an overlap position of about 10xc2x0. The torque then steadily drops off for the remainder of the stroke. For a rapid response, a high starting torque is essential to overcome inertia of components of the rotary actuator and diverter gate. However, a rotary actuator of the xe2x80x9cconstant air gapxe2x80x9d design has a relatively low torque at the end of its operating stroke. This relatively low torque is insufficient to prevent rebound of a diverter gate upon impacting of the diverter gate against a rubber bumper.
The xe2x80x9cdiminishing air gapxe2x80x9d rotary actuator design is characterized by a relatively low starting torque due to large initial air gaps between the rotor and stator pole pieces at the beginning of the operating stroke of the rotary actuator. As the rotor rotates, the air gaps decrease and the torque steadily rises toward a high ending torque. Therefore, for a given power level and loading conditions, the rotary actuators having a xe2x80x9cconstant air gapxe2x80x9d design will produce a higher starting torque than the rotary actuators having a xe2x80x9cdiminishing air gapxe2x80x9d design. However, the xe2x80x9cdiminishing air gapxe2x80x9d rotary actuator design will have a higher end of stroke torque. Although the xe2x80x9cdiminishing air gapxe2x80x9d rotary actuator design has the potential to have a relatively high ending torque, small variations in the final position of the diminishing air gaps, being in a series magnetic circuit arrangement, can result in a large variation in the end of stroke torque of the xe2x80x9cdiminishing air gapxe2x80x9d rotary actuator design.
In the foregoing discussion of the background of the present invention, the rotary actuators have been considered in association with a diverter for mail or other items that are traveling along a conveyor. It should be understood that rotary actuators have and, in all probability, will be used in many different environments. For example, rotary actuators have previously been utilized to actuate valves which control fluid flow.
The present invention provides a new and improved rotary: actuator having a larger beginning of operating stroke torque than is achieved with a corresponding xe2x80x9cdiminishing air gapxe2x80x9d rotary actuator design and a larger ending of operating stroke torque than is achieved with a corresponding xe2x80x9cconstant air gapxe2x80x9d rotary actuator design. This is accomplished by utilizing features of both the xe2x80x9cconstant air gapxe2x80x9d rotary actuator design and the xe2x80x9cdiminishing air gapxe2x80x9d rotary actuator design. Although it is preferred to utilize the improved rotary actuator of the present invention in association with a diverter for mail or other items, it is contemplated that the improved rotary actuator may be utilized in many different environments in association with many different types of devices.
A rotary actuator constructed in accordance with the present invention includes a rotor which is disposed between pole pieces of a stator. The rotor is rotatable relative to the stator between an unactuated position and an actuated position.
A first stator surface on a first pole piece of the stator faces toward and is spaced from a first rotor surface on the rotor by a first working air gap. The first stator surface and the first rotor surface are spaced apart by the same distance when the rotor is in the unactuated position as when the rotor is in the actuated position. Therefore, the axial extent of the working air gap between the first stator surface and the first rotor surface remains constant during rotation of the rotor between the unactuated and actuated positions.
A second stator surface on a second pole piece of the stator faces toward and is spaced from a second rotor surface on the rotor by a second working air gap. The second stator surface and the second rotor surface are spaced apart by a smaller distance when the rotor is in the actuated position than when the rotor is in the unactuated position. Therefore, the axial extent of the second working air gap decreases during rotation of the rotor between the unactuated and actuated positions.
In a further embodiment of the present invention, the configuration of the stator lobes are such that they do not generate any substantial axial force on the rotor shaft. Thus, in one force-balanced embodiment, on a first lobe, the upper surface has a variable spacing configuration and the lower surface has a fixed spacing configuration, and a rotationally adjacent lobe has the lower surface with a variable spacing configuration and the upper surface with a fixed spacing configuration, with the lobe configurations alternating around the rotor. In a preferred force balanced embodiment, each lobe is symmetrical about its plane of rotation between the actuated and unactuated positions although lobe configuration may differ between different lobes. At least one lobe has a variable spacing configuration on its upper and lower surfaces and at least one lobe has a constant spacing configuration on its upper and lower surfaces.